This application claims the priority of 199 54 416.6, filed Dec. 11, 1999, in Germany, the disclosure of which is expressly incorporated by reference herein,
The invention relates to a process for setting into oscillation an electromagnetic actuator, which actuates a switching element, in particular an internal combustion engine lift valve, and in which process an armature, which acts on the switching element, oscillates between two electromagnet coils against the force of at least one reset spring by means of alternating flow around the electromagnet coils. The switching element with the armature and the reset springs represents an oscillatory spring-mass system. Starting from the quiescent state of the system, where the armature is held by the reset springs essentially in the center between these magnet coils, in order to set into oscillation the spring-mass system, the electromagnet coils are excited alternatingly by driving with an electric alternating voltage of a specific frequency. For the technical background reference is made not only to European Patent EP 0 118 591 B1 but also to German Patent DE 33 07 070 C2.
A preferred application for an electromagnetic actuator is the electromagnetically actuated valve drive of internal combustion engines. That is, the gas exchange lift valves of a reciprocating piston internal combustion engine are actuated in the desired manner by such actuators, i.e. oscillatory opening and closing. In such an electromechanical valve drive, the lift valves are moved individually or in groups by means of electromechanical actuating members, referred to as actuators, where the time for opening and closing each lift valve can be chosen virtually arbitrarily. Thus, the valve control times of the internal combustion engine can be optimally adjusted to the current operating state (this is defined by speed and load) as well as to the respective requirements with regard to consumption, torque, emission, comfort and response behavior of a motor vehicle driven by the internal combustion engine.
The essential components of a known actuator for actuating the lift valves of an internal combustion engine are an armature and two electromagnets for holding the armature in the xe2x80x9clift valve openxe2x80x9d or xe2x80x9clift valve closedxe2x80x9d position with the related electromagnet coils, and furthermore reset springs for moving the armature between the xe2x80x9clift valve openxe2x80x9d and xe2x80x9clift valve closedxe2x80x9d positions. To this end, reference is also made to FIGS. 1a and 1b, which depicts such an actuator with assigned lift valve in the two possible end positions of the lift valve and the actuator-armature. Between the two shown states or positions of the actuator-lift valve unit the curve of FIG. 1b shows the armature stroke z or armature path between the two electromagnet coils is plotted over time t in a simplified drawing.
FIGS. 1a and 1c depict the closing operation of an internal combustion lift valve, which is marked with the reference numeral 1 and which moves in the direction of its valve seat 30. As usual, a valve closing spring or first reset spring 2a engages with this lift valve 1. Furthermore, the actuator, which is marked 4 in its entirety, acts on the shaft of the lift valve 1 - here through interconnection with a (not absolutely necessary) hydraulic valve clearance compensating element 3. The actuator includes in addition to the two electromagnet coils 4a, 4b, a push rod 4c, which acts on the shaft of the lift valve 1 and which carries an armature 4d, which can be slid longitudinally so as to oscillate between the electromagnet coils 4a, 4b. Furthermore, a valve opening spring or second reset spring 2b engages with the end of the push rod 4c facing away from the shaft of the lift valve 1.
Thus, it involves a vibratory spring-mass system, for which the valve closing spring 2a and the valve opening spring 2b form a first as well as a second reset spring, and for which consequently the reference numerals 2a, 2b will also be used. FIG. 1a shows the end position of this oscillatory system, in which the lift valve 1 is completely open and the armature 4d rests against the bottom electromagnet coil 4b. FIG. 1c depicts the second end position of the oscillatory system, where the lift valve 1 is completely closed and the armature 4d rests against the upper electromagnet coil 4a. The armature 4d is moved into these two end positions owing to suitable excitation or suppression of the respective magnet coils 4a, 4b. 
In the quiescent state of the system, i.e. when none of the magnet coils 4a, 4b are excited, the armature 4d is located essentially in the center between the two magnet coils 4a, 4b and is held in this position by the suitably designed reset springs 2a, 2b. Starting from this quiescent position, the entire spring-mass system must be set into oscillation for a desired operation of this system, i.e. for a desired oscillating actuation of the lift valve 1.
One possible process for setting into oscillation this spring-mass system is described in the German Patent DE 33 07 070 C2, cited in the introductory part of the specification. Here, too, the electromagnetic actuator with the lift valve and with the reset springs is perceived as a mechanical spring-mass system and the two electromagnet coils are driven with periodic voltage pulses. For the frequency of the exciting periodic voltage pulse the natural frequency of the oscillatory spring-mass system is chosen.
Building on this known state of the art, European Patent first document EP 0 118 591 B1, details that, when viewed over a longer period of time, the exciting frequency be chosen higher than the resonance frequency of the system and then allowed to decrease slowly. In so doing, the frequency is supposed to change slowly so that upon reaching the resonance frequency the lift valve is excited for an adequately long period of time so that the armature can oscillate between its two end positions.
One general drawback with this known state of the art is that, owing to the reciprocal interaction between the electromagnetic part of the actuator and the mechanical spring-mass system, the decoupled approach to these two subsystems that has been practiced to date is only partially reflective of the true situation. Namely this past approach does not take into consideration the intense nonlinear electromagnetic properties of the armature. As a consequence the result is usually many unnecessary preoscillation periods or exciting voltage pulses and thus excess power demand of the electromagnetic actuator. This problem is especially blatant in the state of the art according to European Patent EP 0 118 591 B1, since it is precisely the proposed slow change in the exciter frequency that does not account for a fast change in the resonance frequency owing to the nonlinear effects.
It is an object of the present invention to provide a remedy for this described problem.
The solution to this problem according to the present invention is characterized in that, instead of the hitherto customary essentially rectangular curve of the alternating voltage over the time, an essentially sinusoidal or similar alternating voltage is applied to the electromagnet coils and, in particular, either in the form of a constantly sinusoidal voltage curve or in the form of a correspondingly variable pulse width- modulated voltage curve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.